The billionaire backer of a string of local news websites that spanned New York to Shanghai has sparked outrage by shutting them down just a week after reporters in one newsroom voted to unionise.

In a surprise announcement Joe Ricketts, founder of a discount stock brokerage and Donald Trump backer, closed DNAinfo and Gothamist, sites that chronicled daily life in New York City with local stories. The move leaves 115 journalists out of work, though paid for three more months on “administrative leave”.

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The journalists had “reported tens of thousands of stories that have informed, impacted, and inspired millions of people”, Ricketts said in a statement that replaced the websites. “But DNAinfo is, at the end of the day, a business, and businesses need to be economically successful if they are to endure.”

The closure highlights the financial difficulties local news outlets across the world have faced in recent years amid shrinking advertisement revenue and an environment where sites like Google and Facebook dominate. The Village Voice, another New York publication, stopped its print edition in September after more than 60 years.

But many observers pointed to a decision in the Gothamist newsroom last week to join a union as impetus behind the timing of the closure. Ricketts refused to officially recognise the move, requiring the government to step in.

“The decision by the editorial team to unionize is simply another competitive obstacle making it harder for the business to be financially successful,” a DNAinfo spokeswoman said.

The Writers Guild of America East, the union staff voted to join, said it was “deeply concerned” about the decision to shutter the publications: “It is no secret that threats were made to these workers during the organizing drive.”

Ricketts penned a blogpost in September titled “Why I’m Against Unions At Businesses I Create” where he said: “Unions promote a corrosive us-against-them dynamic that destroys the esprit de corps businesses need to succeed.

“That corrosive dynamic makes no sense in my mind where an entrepreneur is staking his capital on a business that is providing jobs and promoting innovation.”

Current staff were shocked by the news, with Kenneth Tan, managing editor of Shanghaiist, simply tweeting “FML” in response to the announcement.

Ricketts founded DNAinfo in 2009 to cover street-level news in New York City and report “stories that weren’t getting told”, he said. In March the company bought Gothamist, which also covered Washington, Los Angeles, Chicago, San Francisco and Shanghai through a network of sister sites.

“I believe people care deeply about the things that happen where they live and work, I thought we could build a large and loyal audience that advertisers would want to reach,” Ricketts said.

Reaction to the news was a mix of disbelief, sadness and anger.

“Infuriated by what just happened w/ DNAinfo & Gothamist,” activist Shaun King said on Twitter. “They unionized so the asshole billionaire who owned the company shut them down.”